Modeling and Sculpting a Crown
Your first question is not very clear, maybe show a picture of the kind of crown you want to create. If you want to fill a face, select the 4 vertices you want and press F. But instead of extruding 4 distinct vertices, select a top face and extrude it with E then scale down. You can simultaneously extrude several faces as individual faces with alt E, then scale them down with the Pivot Point set to Individual Origin.

For the second question: You can either model or sculpt your ornement. If you choose to sculpt it, the best thing is to keep a low-version of you crown, and once you're good with the sculpted high version, you bake its normals and use the normal map you get to fake 3D on the low-poly. To sculpt I guess you can do it 2 ways: either with the Multiresolution modifier or with Dyntopo. Here is a comparison that might help: sculpting - Everything is edgy and sharp, but I want it smooth